Method of making a sandwich resistor



July 2, 1958 G. c. RIDDLE 3,390,453

` METHOD 0F MAKING: A SANDWICH RlsIsToRl Filed Sept. 24, 1965 AT TORNEYS United States Patent O 3,390,453 METHUD F MAKING A SANDWICH RESIS'IQR Grant C. Riddle, Mountain View, Calif., assigner to International Telephone and Telegraph Corporation, Nutley, NJ., a corporation of Maryland Filed Sept. 24, 1965, Ser. No. 489,897 2 Claims. (Cl. 29-620) ABSTRACT OF THE DISCLOSURE This invention provides a method for obtaining improved high resistance film type resistors in which the desired resistance can be obtained by closely controlling the resistivity and thickness of the thin film resistive material and the area of the for-med resistor.

The present invention is directed to a resistor and more particularly to a sandwich resistor for use with semiconductor integrated circuits and a method of making such resistor.

At the present time, the use of high valued resistors in integrated circuits is limited by the available surface area upon which to deposit a long meandered thin-film resistor. The area of the resistor is proportional to its ohmic value for a given width and, therefore, where it is desired to use a very high valued resistor, for example in the vicinity of hundreds of thousands of' ohms, in order to reduce current .and the amount of dissipated power, the space required for such a resistor is excessive.

Another problem in this field is that with the use of conventional thin film resistors, the thinness of the film, which is directly proportional to the resistance value, is limited by oxidation and the fact that as the film is made thinner, its thickness becomes comparable to the mean free path of conductive electrons in the metal to cause anomalous effects in the resistivity of the material.

Accordingly, it is a general object of the invention to provide an improved resistor for use in semiconductor integrated circuits.

It is another object of the invention to provide a resistor of the type laid down on a semiconductor substrate as a film which requires less area as compared to the prior art.

It is still another object of the invention to provide a film type resistor which is protected against surface contamination.

It is yet another object of the invention to provide a resistor of the above type which eliminates the problem of the thin film resistance effect.

It is yet still another object to provide a film type resistor which is economical in construction.

Itis another object of the invention to provide a method of making a film type resistor in which the value of resistance can be closely controlled.

These and other objects of the invention will become more clearly apparent from the following description when taken in conjunction with the accompanying drawings:

Referring to the drawings:

FIGURE l is a plan view of a resistor constructed in accordance with the present invention;

FIGURE 2 is a cross-section taken along lines 2-2 of FIGURE 1;

FIGURE 3 is an enlarged diagrammatic view of a portion of FIGURES l and 2; and

FIGURE 4 is a block diagram useful in explaining the novel process of the invention.

Referring now to FIGURES 1-3, there is shown the sandwich resistor structure of the present invention. A semiconductor substrate s provided on which is 3,390,453 Patented July 2,1968

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deposited a first conductive strip 11 (such as aluminum). This may be done by any convenient process well known in the art and will, therefore, not be discussed in detail. However, in some applications, the aluminum conductive strip will be deposited on a thin oxide film (not shown) which usually provides protection for the semiconductor substrate 10. Strip 11 has two major surfaces, 11a and 1lb, the first surface 11a being affixed to the substrate 10. A iilm 12 having a predetermined resistivity is in contact with a portion of surface 11b of conductive strip 11. As illustrated, the film is not limited to the strip area but may extend over other portions of substrate 10.

A second conductive strip 13 also having major surfaces 13a and 13b has its surface 13a in contact with film l2, and is juxtaposed over a portion of strip 11 with a predetermined common area 1S of the film between them. This area is delineated by FIGURE 3 and has a length designated l and width designated w, and a thickness t, as shown in FIGURE 3. Conductor 13 is in electrical contact with the top face 15a of the common area, and likewise the other conductor 11 is in contact with the bottom face 15b of the area.

As will be discussed in greater detail below, the resistance between conductors 11 and. 13 is determined by the common area, A, which is the product of l and w, the resistivity p of the film, and its thickness t. In equation form, total resistance between conductors 11 and 13 is It is noted that the common area A is relatively independent of the accuracy of registration of the two conductive strips as long as the strips completely overlap at nearly right angles. The area is, however, highly dependent upon the width of the two strips.

With the above type of resistor, high values of resistance are easily achieved using a minimum area and at the same time having a film of suiicient thickness to eliminate anomalous thin lm effects. Moreover, the electrodes 11 and 13 provide a form of encapsulation for the active part of the film that is serving as a resistor, an action which provides stability and protection. Finally, as is apparent from the above discussion, that since the film resistivity is of such a high value, the resistive film can be deposited as a general area and several pairs of electrode contacts can use portions of the area as needed.

In constructing the device, it is impractical to directly measure the value of the resistor during its deposition because the covering electrode 13 will not be present. Thus, monitoring and computing means are provided for use during the deposition of the film to indicate when to terminate such deposition when the proper value of resistance is reached.

More particularly, there are means provided for monitoring the resistancy of the film, and also its thickness which means are schematically shown in FIGURE 1. These are well known in the art and will not be discussed further. The particular resistancy of the film which is being monitored in this case will be termed the Llateral resistancy. This is the resistance per unit square from the edges of the material (as opposed to its faces 15a and 15b) and is given by Formula 2, resistancy being designated as R;

RI=Q (2) Since this value is measured from the edges of the film being deposited, a value can be monitored almost from the initiation of deposition since the area of the film 12 is unchanging and only its thickness is being varied. The lateral resistancy, R', and thickness, t, values are coupled into a computation circuit or analog computer, illustrated in block diagram in FIGURE 4, which acts on the data to produce a final value of resistance, R, between the conductive strips 11 and 13.

Specifically, the value of p is determined as illustrated in block 16 and Equation 2 by finding the product of lateral resistancy and thickness. The unit area resistancy, R", as illustrated in FIGURE 3, is computed by block 17 according to the relation R"=pt (3) Resistivity p 150 ohm-cm.

Thickness t -5 cm.

Lateral resistance R l5 megohms per square. Unit area resistance R" l.5 1()5 ohm 1.2. Resistor area A it 25n=625n2- Resistor value R 240 ohms.

The resistance value of the silicon of 240 ohms is relatively low for many applications, but other materials, especially the semi-insulating compounds may be utilized, depending on the specific application.

I claim:

1. A process for making a resistor for an integrated circuit comprising the steps of: providing a first conductive strip having a major face; depositing on at least a portion of said major face a film of resistive material; concurrently with said last mentioned step, monitoring the lateral resistancy and thickness of said film and converting said values thereof into electrical signals; feeding said signals into a computer to determine the unit area resistancy between said first strip and the exposed surface of said deposited film; terminating said film deposition when said unit area resistancy reaches a predetermined value; and depositing on said exposed surface of said film a second conductive strip in juxtaposition with said first strip with a predetermined common area of said film therebetween, the total resistance between said strips being determined by the computer by the quotient of said unit area resistancy and said common area.

2. A process for making a resistor for an integrated circuit comprising the steps of: providing a semiconductive substrate; depositing a first conductive strip on said substrate such strip having two major faces one of which is in contact with said substrate; depositing on at least a portion of the other major face of said strip a film of resistive material; concurrently with said last mentioned step, monitoring the lateral resistancy and the thickness of said film and converting said values thereof into electrical signals; feeding said signals into a computer to determine the unit area resistancy between the said other major face of said first strip and the exposed surface of said deposited film; terminating said film deposition when said unit area resstancy reaches a predetermined value; and depositing on said exposed surface of said film a second conductive strip in juxtaposition with said first strip with a predetermined common area of said film therebetween, the total resistance between said strips being determined by the computer by the quotient of said unit area resistancy and said common area.

References Cited UNITED STATES PATENTS 3,050,420 8/1962 Wasserman 29--155.7 X 3,169,892 2/ 1965 Lemelson.

3,220,938 1l/l965 McLean et al.

3,304,471 2/1967 Zuleeg 117--217 X JOHN F. CAMPBELL, Primary Examiner.

I. CLINE, Assistant Examiner. 

